The present invention relates to a method and apparatus for analyzing pulmonary functions and more particularly to such a method and apparatus wherein the functions are analyzed through simultaneous infrared radiation monitoring of a plurality of gases, each selected in accordance with its ability to indicate a separate one of the pulmonary functions.
The analysis of gases being inhaled or exhaled from the lungs has long been employed in the assessment of various pulmonary functions. Possibly the most important or basic of these functions includes the determination of alveolar volume or capacity. This function in itself provides an indication as to the condition of the lungs and in addition is essential for the proper measurement or analysis of additional functions such as pulmonary diffusing capacity and cardiac output as manifested in pulmonary blood flow. However, the techniques employed for monitoring these functions have been time consuming while requiring substantial and generally immobile equipment tending to prevent or impair the ability to fully assess various pulmonary functions. In order to provide a more accurate background for the present invention, the general techniques for assessing each of these functions are described briefly below followed by a discussion of problems presently existing in the prior art which prevent the maximum utilization of pulmonary function analysis.
As indicated above, the measurement of alveolar volume may be employed to assess pulmonary function and in itself provides an index of the severity, or at least changes in severity, of certain patterns of pulmonary function. In addition, an accurate determination of lung volume is essential for the proper understanding of newly recognized patterns of dysfunction which in turn are becoming essential for a number of dynamic (and more substantive) studies of pulmonary function in establishing conditions of health and disease. For example, the measurement of pulmonary diffusing capacity by the single breath method can be no more reliable than the underlying measurement of lung volume. Similarly, the single breath method of assessing pulmonary blood flow is dependent upon the underlying measurement of lung volume.
A simple and commonly known method for measuring alveolar volume consists in causing a patient to inhale a known concentration of an inert and insoluble gas such as helium or neon. After a short selected period of breath-holding to allow uniform distribution of the inhaled gas throughout the lungs, the breath is exhaled and a sample of the exhaled gas is collected and analyzed to determine the concentration of the inert gas, for example by means of a gas chromatograph or mass spectrometer. The resultant determination of the gas dilution ratio between the inhaled and exhaled gases may be employed along with volume determinations of the inhaled and exhaled gases in order to assess the alveolar volume. However, these techniques for measuring alveolar volume have serious disadvantages.
Initially, gas chromatography techniques may take as much as ten minutes to perform under usual pulmonary function laboratory conditions with the analysis being limited to a selected point in time during exhalation. In addition, the need for a gas collection system and chromatograph tends to preclude mobile mass screening programs.
A mass spectrometer provides an effective and portable system. However, as is discussed in greater detail below, the mass spectrometer is itself expensive and commonly requires the use of expensive, rare isotope gases.
The measurement of the diffusing capacity of the lungs has similarly become a useful technique in the diagnosis of pulmonary vascular obstruction, pulmonary fibrosis and subclinical emphysema and accordingly may be used as a screening test for determining general pulmonary condition. In a single breath method for determining diffusing capacity, the subject or patient inhales to vital capacity a low, non-toxic concentration in air of a suitable gas such as low concentration carbon monoxide along with an insoluble and inert tracer gas as described above in connection with the measurement of lung volume.
After the breath is held for a short selected period of for example ten seconds, the subject exhales into a collection means such as a spirometer. The exhaled gas is sampled after a prescribed volume of exhalation with the concentrations of the two test gases in the expirate being determined, for example, by means of gas chromatography.
In this manner, it is possible to calculate alveolar volume as described above by means of the dilution ratio for the inert gas. At the same time, pulmonary diffusing capacity may be determined from the initial concentration of carbon monoxide inhaled into the lungs and the calculated diffusion of carbon monoxide from the lungs between inhalation and exhalation.
In the abovenoted tests for diffusing capacity, helium and neon have been most frequently employed as the insoluble inert tracer gas. Carbon monoxide is particularly effective in determining the diffusing capacity of the lungs because of its ability to combine with hemoglobin much more effectively than oxygen; hence, the rate of diffusion of carbon monoxide from the lungs is limited only by its ability to pass through the alveolar capillary walls. Because of its high affinity for hemoglobin, carbon monoxide is a particularly suitable gas for investigating pulmonary diffusing capacity and diffusion abnormalities. Other gases may possibly be employed in place of carbon monoxide for this function. For example, cyanide compounds tend to exhibit the same tendencies of absorption in hemoglobin while being limited in their ability to pass through the alveolar capillary walls. However, such cyanide compounds tend to be unstable as to instant form. In particular such cyanide compounds tend to be present with equilibrium amounts of the monomer (CN) and the dimer (C.sub.2 N.sub.2) as well as its acid form (HCN). Accordingly, such cyanide compounds are difficult to employ within the abovenoted technique which is normally carried out using carbon monoxide.
In connection with the measurement of diffusing capacity, it is noted that the test is relatively simple, noninvasive and painless but does require a skilled operator and expensive equipment to provide reliable data. Since the method also requires collection of an expired alveolar gas sample for subsequent analysis, the diffusing capacity may be determined only for a single point in the expiration profile. Multiple points during a single breath may be analyzed for example with increased complexity by taking multiple samples. However, reproducibility with such a technique is relatively poor. Accordingly, the sampling requirement within this technique as well as the slow response time of the standard instruments used in the measurements of expired gas concentrations tend to detract from its value as a diagnostic tool in mass screening.
A different but related method for measuring pulmonary diffusing capacity employs a respiratory mass spectrometer with the carbon monoxide being selected as the isotopic form (.sup.13 C.sup.16 O or .sup.12 C.sup.18 O) having the mass numbers of 29 or 30 respectively, to enable ts separation from molecular nitrogen with a mass number of 28, which is identical to that of the common isotope (.sup.12 C.sup.16 O). This method, which permits continuous measurement of the expired gases, enables a determination of the distribution of the diffusing capacity during a single breath and may thus be employed for obtaining more complex diagnostic information. However, it does require expensive equipment and the use of a rare isotopic form gas of limited availability. These factors tend to make the technique prohibitively expensive for routine clinical use.
The abovenoted methods of assessing diffusing capacity also fail to provide a rapid response for the assessment of the amount of carbon monoxide in the exhaled gas at any instant. Accordingly, these methods of measuring diffusing capacity are inadequate for certain diagnostic purposes, in particular, the determination of diffusing capacity during exercise where the rate of diffusion must be instantly monitored across a substantial profile portion of exhalation.
It has also been known that the analysis of respired gases may be employed to measure cardiac output or local pulmonary blood flow. In particular, it has been known for some time that gaseous acetylene in small concentrations may be measured during inhalation and exhalation, also in a single breath method, to determine the rate of blood flow in the capillary walls of the lungs. This test depends upon the ability of the acetylene gas to readily pass through the alveolar capillary walls while having only limited solubility in blood. Accordingly, the rate at which acetylene is internally absorbed by the body from the lungs depends upon the rate at which blood is made available for its absorption. Here again, acetylene is used in combination with an inert gas of the type discussed above to permit a simultaneous assessment of alveolar volume in order to calculate the actual rate of loss of acetylene from the lungs as determined by pulmonary blood flow.
The abovenoted technique of pulmonary analysis for assessing blood flow has been used only to a limited extent because of the more developed and thus more commonly employed technique, at least to date, of cardiac catheterization based upon oxygen absorption. However, pulmonary testing with acetylene is of particular advantage in that it is noninvasive and may be adapted to provide an instantaneous measurement of blood flow through the gas exchanging surfaces of walls in the lungs.
It may be seen that various techniques are presently known for accomplishing the determination of alveolar volume as well as the determination of pulmonary diffusing capacity and pulmonary blood flow, for example. However, it has not heretofore been possible to rapidly accomplish various combinations of these determinations with simple portable equipment having a rapid response for assessing the particular pulmonary functions across the entire expiration profile.
There has thus been found to remain a need for a method and apparatus for more rapidly and conveniently assessing pulmonary functions such as those noted above in order to permit the use of such tests in routine clinical use for example as a screening tool.